In an attempt to provide composite materials that have a combination of desirable properties, especially a combination of excellent thermalshock resistance, oxidation resistance and high fracture toughness of silicon carbide, with the hardness, wear resistance and low specific gravity of boron carbide, articles have been made from mixtures of silicon carbide and boron carbide. By varying the proportions of silicon carbide and boron carbide, it should be possible to manufacture articles from mixtures of silicon carbide and boron carbide which have properties required for certain uses that is, for a specific use.
Attempts to manufacture articles from boron carbide/silicon carbide composites were reported as early as 1962. In the early work, mixtures of silicon carbide with from 10 to 90% by volume of boron carbide were hotpressed in graphite moulds under a pressure of 7000 psi (48.2 MPa) at a temperature of 2100.degree. C. Articles having densities of from 93.6 to 96% of the theoretical density (hereinafter abbreviated as % TD) were made by the process (see W. R. Jacoby et al. in "Neutron Absorber Materials for Reactor Control", chapter 4.6 c, pages 221-222, edited by W. Kermit Anderson and Y. S. Theilacker, Naval Reactors, Division of Reactor Development, United States Atomic Energy Commission [1962]). In a similar manner, SiC/B.sub.4 C plates containing 20 or 50% by volume of B.sub.4 C, having a density of 3.03 or 2.86 g/cm.sup.3, respectively, corresponding to 99 or 100% TD, respectively, have been manufactured from a mixture of silicon carbide powder having an average particle size of 5 .mu.m and boron carbide powder having an average particle size of 2 .mu.m, by hot pressing in graphite moulds at a pressure of 5000 psi (34.5 MPa) and a temperature of 2200.degree. C. (see R. G. Gray and L. R. Lynam, Technical Report WAPD-261 [1963].
Self-bonded SiC/B.sub.4 C sintered articles that is, articles which have no additional ceramic or metallic binders as accompanying phases, can be produced with very low porosity by the hot pressing or pressure sintering process. Articles having substantially zero porosity can be produced.
The ability to form articles with complex shapes by pressure sintering is limited. Only relatively small and geometrically simple articles can be produced by the process. Furthermore, the process is associated with very high usage of energy and moulding material. Because the composite material is very hard, final machining of hot-pressed shaped articles made from mixtures of silicon carbide and boron carbide must be done with diamond tools, which is expensive and time consuming.
Attempts have been made to replace the expensive hot-pressing process by the pressureless-sintering process, for manufacturing articles from a composite material comprising silicon carbide and boron carbide. The pressure-less-sintering process permits the manufacture of articles having complex shapes without the need for expensive final machining operations and makes possible mass-production and continuous operation.
The pressureless sintering of a SiC/B.sub.4 C system was reported by S. R. Billington et al. in "Special Ceramics 1964", pages 19-34, edited by P. Popper, British Ceramic Research Association, Academic Press, London and New York [1965]. The examination of articles which had been sintered from .alpha.-SiC with additions of from 10 to 30% by weight of B.sub.4 C at temperatures of from 2200.degree. to 2300.degree. C. were reported by Billington et al. They reported that sintering occurred via formation of a eutectic liquid phase in conjunction with a large volume shrinkage, but that poor densification was achieved (maximum density: 2.65 g/cm3, corresponding to 87.7% TD in the case of a 30% by weight addition of B.sub.4 C).
Non-porous articles of SiC/B.sub.4 C can be obtained, however, by infiltration of an eutectic melt of B.sub.4 C/SiC into pre-shaped articles of .alpha.-SiC with subsequent solidification in a temperature gradient (see U.S. Pat. No. 3,852,099). It is very difficult to carry out that process. Owing to the differing thermal expansion of the SiC matrix and the B.sub.4 C/SiC eutectic articles made by the process tend to form microcracks and have only limited strength.
In contrast, it is possible, using pressureless sintering, to obtain sintered polycrystalline articles comprising silicon carbide, boron carbide and free carbon having a density of at least 85% TD (see U.S. Pat. No. 4,081,284, which corresponds to DE-OS No. 27 34 425). These articles consist of .alpha.-SiC and/or .beta.-Sic and free carbon, said silicon carbide having a uniform grain size of less than 10 .mu.m, said boron carbide being present in an amount of 10 to 30% by weight and being uniformly dispersed throughout said bodies in the form of fine grains, said free carbon being present in an amount of 0.001 to 1% by weight in the form of particles submicron in size.
In the process, homogeneous pulverulent mixtures of submicron particles of .beta.-SiC, from 10 to 30% by weight of B.sub.4 C, and a carbon-containing additive in an amount corresponding to 0.1 to 1% by weight of free carbon, were pre-shaped to form green bodies which were subsequently subjected to pressureless sintering in an inert atmosphere at temperatures of from 2000.degree. C. to below the melting point of the B.sub.4 C/SiC eutectic.
It is apparent from the disclosure that the amount of boron carbide used, which is in the range of from 10 to 30% by weight, based on the total weight of SiC and B.sub.4 C, is critical for achieving the desired density and fine-grained microstructure in the finished sintered articles. With less than 10% by weight of B.sub.4 C, the grain growth can not be controlled during sintering and large plate-like SiC crystals are formed and fine-grained microstructure, which is necessary for mechanical strength, can not be obtained. When more than 30% by weight of B.sub.4 C is present in the composition, the density falls below 85% TD.
Amounts of free carbon, in the pulverulent starting mixture, significantly in excess of about 1% by weight, based on the total weight of SiC and B.sub.4 C, are disclosed but do not provide any significant advantage and function much like permanent pores in the sintered article, in the sense of impairing the ultimate achievable density and strength.
The relationship between percent theoretical density and the amount of B.sub.4 C additives mixed with the SiC and sintered at from 2080.degree. to 2090.degree. C. is shown in FIG. 2. It can be seen that a maximum of 97% TD was achieved with a B.sub.4 C addition of 11.3% by weight, however, when more than 12% by weight of B.sub.4 C was in the mixture, the % TD of the resulting B.sub.4 C/SiC sintered articles decreased.
According to the known pressureless-sintering process, sintering densities of more than 95% TD can be achieved only with B.sub.4 C additions within a very limited range of from approximately 10 to 20% by weight, based on the total weight of SiC and B.sub.4 C. Because of high residual porosity, SiC/B.sub.4 C sintered articles having densities of less than 95% TD are less resistant to oxidation and less resistant to wear and no longer have the desired combination of properties of SiC and B.sub.4 C. Furthermore, the residual porosity adversely affects the mechanical properties. Our own tests have shown that the flexural strength of SiC/B.sub.4 C sintered articles having a density of only 90% TD, does not exceed 300N/mm2, and decreases considerably at increased test temperatures.
Furthermore, the submicron sized SiC powders utilized in the known process have been single-phase, or substantially (.gtoreq.99%) single phase beta-SiC. Beta-SiC, which has a cubic crystalline structure, is a low-temperature form of silicon carbide and is more difficult to produce and potentially more expensive than the alpha(non-cubic) silicon carbide.